Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations, and then purge the stored vapors during a subsequent engine operation. In an effort to meet stringent federal emissions regulations, emission control systems may need to be intermittently diagnosed for the presence of leaks that could release fuel vapors to the atmosphere.
Evaporative leaks may be identified using engine-off natural vacuum (EONV) during conditions when a vehicle engine is not operating. In particular, a fuel system may be isolated at an engine-off event. The pressure in such a fuel system will increase if the tank is heated further (e.g. from hot exhaust or a hot parking surface) as liquid fuel vaporizes. As a fuel tank cools down, a vacuum is generated therein as fuel vapors condense to liquid fuel. Vacuum generation is monitored and leaks identified based on expected vacuum development or expected rates of vacuum development. The EONV leak test may be monitored for a period of time based on an available battery charge.
However, the EONV leak test is prone to false failures based on customer driving and parking habits. For example, a refueling event that fills the fuel tank with relatively cool liquid fuel followed by a short ensuing trip may fail to heat the fuel bulk mass and result in a false fail if an EONV test is run. Further, the rates of vacuum development are based in part on the ambient temperature. During mild weather conditions, the ambient temperature may restrict the amount of heating or cooling of the fuel tank following engine shut-off, and thus limit the rate of pressure or vacuum development. As such, vacuum may not reach expected threshold levels in the time allotted for the EONV test based on available battery charge. This may result in a false-fail condition, leading to potentially unnecessary engine service.
The inventors herein have realized the above issues, and developed systems and methods to at least partially address the problems. In one example, a method for a vehicle fuel system, comprising: during an engine-off condition, including an ambient temperature within a threshold range, operating a cooling fan to increase a fuel tank vacuum; and indicating leaks in the vehicle fuel system based on the increased vacuum. In this way, an EONV test may be run at mild ambient temperatures which would not otherwise result in the development of adequate fuel tank vacuum to pass the EONV test. Implementing this method increases the robustness of the EONV test.
In another example, a method for a vehicle fuel system, comprising: sealing a fuel tank and monitoring a fuel tank pressure after an engine-off event; and in response to a change in fuel tank pressure during a monitoring period being less than a fuel tank pressure threshold, unsealing the vehicle fuel system and allowing the fuel tank pressure to equilibrate to atmospheric pressure; assessing an ambient temperature; in response to the ambient temperature being within a threshold range, sealing the vehicle fuel system; and operating a cooling fan to increase a fuel tank vacuum to identify leaks in the vehicle fuel system. In this way, the occurrence of false EONV test failures due to insufficient vacuum developing may be reduced. This in turn, will reduce unnecessary vehicle warranty care.
In yet another example, a vehicle system, comprising: an engine including an intake; a fuel system including a fuel tank, a fuel vapor canister, a first valve coupling the fuel vapor canister to the intake, and a second valve coupling the fuel vapor canister to atmosphere; a pressure sensor coupled to the fuel tank for estimating a fuel tank pressure; a temperature sensor externally coupled to the vehicle system for estimating ambient temperature; an engine cooling system including one or more cooling fans; and a control system including executable instructions stored in non-transitory memory for: during an engine-off condition, in response to an ambient temperature being within a threshold temperature range; closing the first valve and the second valve; activating the one or more cooling fans; and monitoring a fuel tank vacuum for a vacuum test duration to identify leaks in the fuel system. In this way, EONV test restrictions based on stored battery charge may be avoided. EONV tests may be limited to monitoring vacuum development for a duration (e.g. 45 minutes) in order to maintain sufficient battery charge to restart the vehicle. This system increases the chances that a sufficient vacuum will develop following engine shut-off to correctly assess the presence of fuel system leaks within the testing duration.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.